1. Field of the Invention
The present invention relates to a printing mode shifting device for offset printers wherein blanket to blanket type printing with four cylinders excluding an impression cylinder in contact with one another in series and satellite type printing with five cylinders including the impression cylinder in contact with one another in series are carried out, comprising a means for shifting the first printing mode to the second printing mode, and vice versa, the shifting means permitting a pair of blanket cylinders to be displaced with respect to each other and with respect to the other cylinders, the shifting means also permitting the pair blanket cylinders to be separated from each other and from the other cylinders so as to set the pair of cylinders in intermediate positions, i.e. intermediate throw-off positions.
2. Description of the Prior Art
In conventional offset printers wherein both blanket to blanket type printing and satellite type printing are carried out, the shifting of the blanket to blanket type printing mode to the satellite type printing mode cannot be completely carried out by displacing the pair of blanket cylinders alone. It requires another means which is not operatively connected to a means for displacing the pair of blanket cylinders. Such an additional means consists of a means for axially displacing one of two gears for the impression cylinder and one of a pair of gears for the pair blanket cylinders (refer to FIGS. 7 and 8 of U.S. Pat. No. 3,329,086), or a means for displacing an impression cylinder mechanism including a gear therefor toward and away from the pair of blanket cylinders (refer to FIGS. 9 and 10 of the above-mentioned patent). Therefore, it is troublesome to control such an offset printer, and control errors possibly occur. When control errors occur, the driving system may be greatly damaged.
If the printing modes can be shifted from one to the other by merely displacing the pair of blanket cylinders, and if the driving systems for the pair of blanket cylinders are also automatically shifted in accordance with the displacement of the blanket cylinders, a desired printing mode is obtained by one action so that a mechanism for engaging and disengaging driving gears can be omitted. This allows the construction of an offset printer of this kind to be simplified, and control errors never occur.
The above-mentioned drawbacks encountered in a conventional printing mode shifting device resides in the fact that it is strictly necessary in the shifting device that the amount of displacement of a pair blanket cylinders between themselves and the amount of displacement of the pair of blanket cylinders between these blanket cylinders and an impression cylinder is small so that it does not exceed the tooth height of each gear referred to above. The necessity of setting the amount of displacement of the pair of blanket cylinders to such a low level resides in the following. In offset printing, it is necessary that throw-off positions where the pair of blanket cylinders are separated from each other and from any other cylinders be provided between an ultimate position for conducting blanket to blanket type printing and an ultimate position for conducting satellite type printing. It is necessary in a throw-off operation that the gears in driving systems be properly engaged with one another so that the transmission of power by the driving system for either blanket to blanket type printing or satellite type printing can be effected. If the gears are not engaged with one another properly in such a case, backlash necessarily occurs so that the power transmitting functions thereof are adversely affected.
The first problem referred to above may be solved by providing a method of increasing the amount of displacement of the pair of blanket cylinders to such an extent that it exceeds the tooth height of each driving gear. However, such an increase in the amount of displacement of the pair of blanket cylinders does not ensure a comparatively tight engagement of gears in driving systems in a throw-off position.